1. Field of the Invention
The invention relates to phase modulators which are operated between two phase states, and more particularly to a phase modulator suitable for use in a frequency multiplication chain to produce an in-phase state (i.e. at 0.degree. reference phase) and an out of phase state (i.e. 180.degree. from reference phase) upon a carrier wave, or equivalently .+-.90.degree. from reference phase.
2. Prior Art
Phase modulators have been used for some time to provide phase coding in a transmitted wave. In radar systems, the phase modulation will assume an in-phase state, corresponding to a "1" and an out of phase state corresponding to a "0". (The two phase states may also be regarded as plus or minus 90.degree. from a reference phase state.) The duration of one bit of the phase coding is typically many cycles, 200 nanoseconds in a typical system, and it is desirable that the change in phase state take place in a relatively short duration, typically 20 nanoseconds.
In a typical phase modulation system, there is frequently a very significant variation in amplitude during the transition from one phase state to the other. In a known bi-phase modulator circuit, the amplitude of the carrier is reduced 23 dB in the transition from one phase state to the other phase state, and then reduced again by 23 dB in the transition from the other phase state back to the original phase state.
This very substantial amplitude modulation produces a number of undesirable side effects in the utilization circuitry, and for that reason efforts have been directed to reducing the amplitude modulation to a much lower level. One such approach has been to allow the phase modulation to produce a small fraction of the .+-.90.degree. phase modulation on a low frequency carrier, after which the carrier is multiplied by the reciprocal of the fraction to achieve .+-.90.degree. phase modulation in the final high frequency carrier. In a known example, reducing the phase modulation to .+-.45.degree. and multiplying the frequency by a factor of two reduces the amplitude variation to approximately 1.5 dB. Reducing the phase modulation angle to approximately .+-.22.5.degree. reduces the amplitude modulation to approximately 0.6 dB and requires that the frequency be multiplied by a factor of four.
The approach of reducing the phase modulation angle and multiplying the frequency to get .+-.90.degree. phase modulation at the final carrier frequency with rectangular phasor combinations is accordingly useful, however these combinations can be modified using some of the same components to reduce the residual ripple even more.